1. Field of the Invention
This invention relates to ion sources, and, more particularly, to alloys for liquid metal ion sources.
2. Description of the Prior Art
High brightness sources are sources with a small virtual source size and emit ions at high current density. There are two types of field-ionization sources that offer the high brightness needed for focusing ion beams to a submicrometer spot size--the gas field ion (GFI) source and the liquid metal (LM) ion source. The two sources differ in the mechanism used to feed atoms into the ionizing region. In the field-emission microscope, developed during the early 1950's, the ion source is based on gas field ionization in the immediate vicinity of an extremely shart submicrometer-radius point. The GFI source has an extremely small virtual source size--typically less than 100 A. Since the source of ions is a gas at a pressure of a few micrometers of Hg, the number of atoms available for ionization is limited by the arrival rate of atoms in the vicinity of the small needle. The maximum gas pressure is limited by electrical breakdown of the gas. Since the process occurs on or near a very small surface, a surface that is subjected to a very high electrical field (about 1 V/A) and to very high mechanical stress, this type of source is very sensitive to contaminants and sputtering effects that might dull the needle
A recent breakthrough in high-brightness sources was the exploitation of the LM type of field-ionization source by Clampitt in U.S. Pat. No. 4,088,919. In this source, a relatively dull needle is covered by a layer of liquid metal such as an alkali metal. The application of a strong field causes the liquid metal to form a cusp, and this cusp becomes the emitting point of the source, emitting alkali ions such as lithium. The salient advantage of the LM source is its electrostatically shaped point, which is relatively insensitive to contaminants and such effects that might dull a solid metal point. The source is also notable for an ability to deliver relatively high current.
Other liquid metal sources are discussed in U.S. Pat. No. 3,579,011, which employs mercury as the liquid metal, and U.S. Pat. No. 3,475,636, which discloses a forced-flow, gravity-independent liquid metal arc cathode that includes a pool-keeping structure and is provided with a passage way means for feeding liquid metal to the pool-keeping structure. These two patents are described as being useful in ion thruster applications. U.S. Pat. No. 3,475,636 discloses employing mercury, gallium and cesium in a liquid metal arc cathode. All of these references, however, disclose electron sources, not ion sources.
There are applications requiring either the generation of multiple ionic species or the generation of an ionic species, the element of which has such a high vapor pressure in its liquid state that it is unsuitable for use in the systems described above.
The requirement for a liquid is basic to the LM source--the material to be ionized must be a liquid and it should have a reasonably low vapor pressure in the liquid state. Excessive thermal evaporation of atoms from the source would tend to both contaminate the system and deplete the source.